The present invention relates to a transparent conductive film used in an electrode of solar cells, liquid crystal display devices, flat panel displays, organic electroluminescence devices and the like. More specifically, the present invention relates to the transparent conductive film using zinc oxide that has the resistance to a reducing atmosphere and is a stable material against the high temperature.
Conventionally, ITO (Indium Tin Oxide), indium oxide or the like has been used in the transparent conductive film used as the electrode of the solar cells, the liquid crystal display devices and the like. Since the electrode of these is required not only to be low in the electrical resistance but also to transmit the light without loss, the foregoing material is used.
It is known that, however, in In used in ITO or indium oxide, there are problems of supplying the material due to the exhaustion of indium resources, of instability in the reducing gas and of deterioration due to the operation in the high temperature. Alternative substances are therefore being actively studied, and zinc oxide is noted among others since it has properties superior to ITO in the respects of the resistance to the reducing gas and the stability in the high temperature.
This ZnO has the transparency and has no problem with regard to the transmittance of light, but, with regard to the conductivity, the resistivity of ZnO remains on the order of 10xe2x88x924 xcexa9xc2x7cm though conductivity thereof is yielded by doping the element of Group III such as Ga or Al, whereas the resistivity of ITO reaches the order of 10xe2x88x925 xcexa9xc2x7cm. That is, the value of 7.7xc3x9710xe2x88x925 xcexa9xc2x7cm was reported as the resistivity of ITO, for example, in xe2x80x9cHighly electrically conductive indium-tin-oxide thin film epitaxially grown on yttria-stabilized zirconia (001) by pulse-laser depositionxe2x80x9d by H. Ohta et al. (Applied Physics Letters vol. 76, pp. 2740-2742, 2000), and the value of 2.6xc3x9710xe2x88x924 xcexa9xc2x7cm was reported in the resistivity of ZnO, for example, in xe2x80x9cTransparent and conductive Ga-doped ZnO film grown by low pressure metal organic chemical vapor depositionxe2x80x9d by Y. Li et al. (Journal of Vacuum Science Technology A15, pp. 1063-1068, 1997).
As described above, though ZnO has the advantage as the material, it has not yet become commercially practical as the transparent film for the electrode because, in ZnO, the resistivity which is the most important characteristic is large. The present inventors have deeply researched to lower the electrical resistivity of the zinc oxide, and therefore found that, in the case of ITO, the transparent film for electrode has as high mobilities as 40 to 50 cmxe2x88x922Vxe2x88x921sxe2x88x921 even when the carrier concentrations are as high as 1021 cmxe2x88x923 but, in the case of ZnO, the mobility similar to the ITO can not be attained at the same carrier concentration as described above. It is thought that the reason is that a large amount of donor atoms ionized are produced and the ion scattering thereof occurs when a large amount of elements of Group III are doped to the transparent film for the electrode. On the other hand, when the equivalent of the reduction of the mobility is supplemented with the carrier concentration, the problem that the transparent film for the electrode becomes metallic due to the excess dopant and lose the transparency arises.
The present invention has been invented considering these situations, and it is an object of the present invention to provide a transparent conductive film of ZnO having the electrical resistivity lower than ITO by lowering the electrical resistivity.
The present inventors have deeply researched to lower the electrical resistivity of Zn oxide, and therefore found that it is possible to obtain the n-type ZnO layer being very low in the resistivity by doping nitrogen gradually to the ZnO while doping a definite amount of Ga and completed the present invention. That is, in the transparent conductive film of zinc oxide in accordance with the present invention, the zinc oxide layer is doped with an n-type dopant and a p-type dopant as dopants, and the n-type dopant is more than the p-type dopant and doped into the zinc oxide layer in an impurity concentration of 1xc3x971018 cmxe2x88x923 or more.
By forming this structure, combined substance of an acceptor and an donor such as Gaxe2x80x94Nxe2x80x94Ga is formed in the ZnO layer, and since the combined substance has a strong covalent energy, a donor level of Ga becomes shallower due to interaction and an activation rate of Ga is increased. Consequently, even when the amount of Ga doped is less, carrier concentration may be earned, and therefore the resistivity may be lowered. That is, since the amount of Ga as a dopant to achieve the carrier concentration to be aimed may be reduced, it is possible to reduce an amount of Ga atoms ionized and to reduce the effect of ion scattering. Furthermore, since Ga and N enter into ZnO in a state of bonding, it is possible to neutralize the ionized Ga electrically. Therefore, it is possible to decrease an ion scattering factor due to the ionized Ga and to raise the mobility of the carriers.
The foregoing n-type dopant and the foregoing p-type dopant are doped into the zinc oxide layer in such a way that the ratio of the n-type dopant to the p-type dopant becomes (1.3-3):1, and therefore the both impurities are doped in a stable state.